1. Field of the Invention
The present invention relates to a rubber composition for a tread of a pneumatic tire. More particularly, it relates to a rubber composition for a tread of a pneumatic tire capable of simultaneously satisfying the wet skid resistance, rolling resistance, and wear resistance.
2. Description of the Related Art
Along with the social needs for energy conservation and resource conservation in automobiles, research for reducing the rolling resistance of tires for improving the fuel economy of automobiles has become more important in the tire industry. It is well known that the fuel consumption can be lowered by decreasing the rolling resistance of the tire.
To decrease the rolling resistance of tires, it is effective to use rubber with a small hysterisis loss for the tread rubber. However, use of a rubber with a small hysterisis loss tends to lower the skid resistance on wet road surfaces, that is, the wet skid resistance. These two characteristics are in a contradictory relationship. Therefore, it has been difficult in the past to simultaneously satisfy the two characteristics of the rolling resistance and wet skid resistance in a tire.
Recently, the correspondence between the rolling resistance and wet skid resistance of tires and the viscoelasticity of the rubber composition has been theoretically demonstrated. It has been shown that to decrease the rolling resistance at the time of tire use, and for low fuel consumption, it is effective to decrease the hysterisis loss of the tread rubber, that is, to decrease the dynamic loss (i.e., tan .delta.) at a temperature (i.e., running temperature) of 50.degree. to 70.degree. C. Contrary to this, the wet skid resistance is closely correlated viscoelastically with the dynamic loss at low temperatures, in actuality, close to 0.degree. C., since the frequency of deformation for following the surface roughness of the road is large. Accordingly, improvement of the grip performance of a tire requires that the tan .delta. near 0.degree. C. be made larger.
As a method for decreasing the hysterisis loss, it is known to be effective to use rubber with a low glass transition temperature, for example, a high cis-polybutadiene or natural rubber. This decreases the tan .delta. at 0.degree. C., however, so the wet skid resistance is decreased and it is difficult to obtain both a high wet skid resistance and a low rolling resistance.
In recent years, numerous proposals have been made on inventions satisfying the above contradictory properties based upon the advances in solution polymerization of SBR and BR. For example, Japanese Unexamined Patent Publication (Kokai) No. 55-12133 and Japanese Unexamined Patent Publication (Kokai) No. 56-127650 propose a high vinyl polybutadiene rubber, while Japanese Unexamined Patent Publication (Kokai) No. 57-5520 and Japanese Unexamined Patent Publication (Kokai) No. 57-73030 proposes a high vinyl styrene-butadiene copolymer rubber.
Further, Japanese Unexamined Patent Publication (Kokai) No. 59-117514, Japanese Unexamined Patent Publication (Kokai) No. 61-103902, Japanese Unexamined Patent Publication (Kokai) No. 61-14214, Japanese Unexamined Patent Publication (Kokai) No. 61-141741, etc. propose to decrease the heat buildup by the use of a modified polymer introducing a functional group such as benzophenone or isocyanate into the molecular chain of the polymer. Further, Japanese Unexamined Patent Publication (Kokai) No. 3-239737 shows that it is possible to achieve a high wet skid resistance, low rolling resistance, and high wear resistance at the same time by using a styrene-butadiene copolymer rubber (SBR) having a specific styrene chain.
Further, Japanese Unexamined Patent Publication (Kokai) No. 3-252431 proposes a combination of an SBR having a specific styrene chain and a silane coupling agent, while Japanese Unexamined Patent Publication (Kokai) No. 3-252433 proposes to achieve the three characteristics of the wet skid resistance, rolling resistance, and wear resistance by causing a reaction between terminal-modified SBR and silica.
However, none of these proposals satisfies the above three characteristics in the performance thereof.